Conventionally, a semiconductor device for surge protection using a constant voltage diode is known. In such a semiconductor device for surge protection, when a surge voltage (overvoltage) as high as a breakdown voltage of a constant voltage diode is applied, for protection of an apparatus, a surge current (overcurrent) that flows due to the surge voltage is caused to flow in a ground direction by the constant voltage diode (see, for example, Patent Document 1).
FIG. 4 is a cross-sectional view of a conventional semiconductor device for surge protection proposed in Patent Document 1 described above. As shown in FIG. 4, the semiconductor device for surge protection includes a semiconductor substrate 100. The semiconductor substrate 100 includes a high concentration n-type semiconductor substrate 110, a low concentration n-type semiconductor layer 111 that is formed on the high concentration n-type semiconductor substrate 110, a p-type semiconductor layer 112 that extends from a surface of and into the low concentration n-type semiconductor layer 111, and a high concentration n-type semiconductor layer 113 that extends from a surface of and into the p-type semiconductor layer 112. Further, an insulating film 116 is formed so as to cover the surface of the low concentration n-type semiconductor layer 111, the surface of the p-type semiconductor layer 112, and a portion of a surface of the high concentration n-type semiconductor layer 113 other than a surface center region. Moreover, a cathode electrode 114 is formed so as to span the surface center region of the high concentration n-type semiconductor layer 113, which is not covered with the insulating film 116, and a portion of the insulating film 116. Further, an anode electrode 115 is formed so as to cover a surface of the high concentration n-type semiconductor substrate 110. Reference characters J11, J12 and J13 indicate an interface between the insulating film 116 and the low concentration n-type semiconductor layer 111, an interface between the insulating film 116 and the p-type semiconductor layer 112, and an interface between the insulating film 116 and the high concentration n-type semiconductor layer 113, respectively. Further, reference characters J14 and J15 indicate an interface between the low concentration n-type semiconductor layer 111 and the p-type semiconductor layer 112 and an interface between the high concentration n-type semiconductor layer 113 and the p-type semiconductor layer 112, respectively. This semiconductor device for surge protection has an effect of, when a surge voltage as high as a breakdown voltage is applied across the cathode electrode 114 and the anode electrode 115, causing a surge current that flows due to this surge voltage to flow to the anode electrode 115 side without flowing to an apparatus (not shown) side.
Patent Document 1: JP 2003-110119 A
However, the above-described semiconductor device for surge protection has presented a problem that, in the vicinity of an interface between the semiconductor substrate 100 made of silicon or the like and the insulating film 116 made of silicon oxide or the like, crystal defects are present that are attributable to a difference in crystal structure such as in inter-lattice distance between the semiconductor substrate 100 and the insulating film 116, resulting in the semiconductor device having low surge resistance (resistance to a surge voltage). That is, in the semiconductor device for surge protection, the interfaces J15 and J14 extend to reach the insulating film 116, which leads to the fear that, upon application of a surge voltage, a surge current may flow through the interfaces J11, J12 and J13 where surge resistance has been deteriorated due to crystal defects.